No it's not. Maybe about 5 miles across and 20 minutes without traffic. But that 20 minutes can easily become 40 minutes or worse, and if your plans/activities involve a lot of Centro amenities (it probably offers the most of any one neighborhood) then you might find yourself wanting to come and go
a few times a day. That would get inconvenient pretty quickly.

If, on the other hand, you just want a taste of various things and to check out a few different neighborhoods then Polanco is fine. Most of the tourist stuff and the nicer, upmarket, expat type of locations/services are generally clustered in the same region, from Polanco down to Napoles and across to Centro.